Searching for oil or, more generally, hydrocarbons has become more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper or in places difficult to reach and below the sea. Prospecting for and exploitation of hydrocarbon fields demands hardware which is more resistant to environmental challenges such as higher loads and corrosion, which were less important previously.
In some applications threaded joints can be subject to deformation of the joint seals.
Modern joints are generally designed with metal to metal seals made by the contact between two surfaces, usually at one end or at both ends of the threaded portion of the joint, interfering in the elastic range of the modulus of elasticity for an appropriate stress magnitude. However there are situations where resilient seals are needed instead of or in combination with metal seals, to prevent penetration of external fluids into the interstices of the threads.
It is therefore a design requirement that the joint seals resist penetration of external or internal fluids, or at least do not allow continuous exchange of fluids that have already penetrated the joint with surrounding fluids, in order to reduce the corrosion rate.
Currently a widespread technical solution to the problem of externally sealing a threaded connection is to use O-rings or resilient seal rings of various cross-sections, made of elastomeric or composite materials.
Complex resilient seal rings and simple O-rings perform their sealing function based on the diametrical geometric interference between pin and box, which is predefined with respect to at least one of the joint members. Said geometric interference appears after make up of the connection, to elastically deform the seal ring and thus induce contact pressures between the seal and each of the pin and box, defining a mechanical barrier which seals the joint. An additional energization of the seal ring is provided by the external fluid pressure which increases deformation and adherence to the seat where the seal ring is housed.
An example of an O-ring is disclosed in U.S. Pat. No. 6,173,968 for sealing a joint between a pin and box. An O-ring abuts an annular backup ring of substantially the same diameter. The annular backup ring is split to permit radial expansion and has a greater thickness on its outer periphery than on its inner periphery. When the joint being sealed is under high pressure, the seal ring urges the backup ring to expand radially to cover any gap between the members being sealed, maintaining sealing capacity, even under high temperature conditions, and preventing the sealing ring from extruding into the gap. The pressure varies with sea depth and seal efficiency is reduced when lower pressures act on the O-ring.
In this document the external pressure on the joint determines also the pressure acting on the O-ring. When higher contact pressures are needed for the O-ring, then higher geometric interference is required between the O-ring and joint members. This might cause seal breakage.
Another way to improve efficiency of the sealing capacity of the O-ring is by increasing geometric interference, which is achieved in most cases by making the seal ring radially bigger than its housing. However, the bigger the seal ring, the more exposed is it to damage during make up, especially when it is pre-mounted in the box member and it is forced to overcome the entire pin threaded area.
In this case other drawbacks may arise. Several geometric connection variables that originate during the manufacturing process, such as ovality, eccentricity, and rugosity, introduce uneven interference over the whole circumference of the sealing surfaces, thus producing uneven contact pressures and reducing the sealing capacity.
In practice, sealing capacity due to geometric interference is limited by the geometry, mainly radial sizes and length, of resilient elements in relation to their ability to be dragged during make up across the threads and any other interfering surface without being damaged.